(1) Field of the Invention
The present invention relates to a charge detecting apparatus for detecting charge completion time when charging a secondary battery in which a electrolyte is used.
(2) Description of the Related Art
Conventional methods of charging secondary batteries are broadly classified into constant-voltage charging and constant-current charging. Methods of detecting charge completion time include detection of terminal voltage, detection of terminal voltage variations and detection of a battery internal pressure.
The constant-voltage charging is not suited for rapid charging in that, although a battery may be charged by means of a large quantity of current in an initial stage of charging, only a small quantity of current may be used in later stages. The method suited for rapid charging is the constant-current charging which allows a large and constant quantity of current to be supplied from beginning to end of charging.
However, the constant-current charging always requires some means to prevent an overcharge. It is therefore necessary to check whether a battery is fully charged or not. Broadly, there are the following three methods for determining the fully charged state of a battery:
(1) "Terminal Voltage Detection"
This is a method in which charging is regarded as being completed when the terminal voltage of a battery being charged reaches a predetermined level.
(2) "Detection of Terminal Voltage Variations"
This method detects a voltage between the terminals of a secondary battery, and charging is regarded as being completed when the gradient (time variation rate) of this voltage reaches a predetermined level.
(3) "Detection of Battery Internal Pressure"
This method utilizes bubbles produced internally of a battery when charging is completed, and charging is regarded as being completed when the internal gas pressure of a battery having a special, enclosed structure reaches a predetermined level.
Features of these methods will be described hereunder.